J. E. S. Costa

SDSS DR7 WHITE DWARF CATALOG

We present a new catalog of spectroscopically confirmed white dwarf stars from the Sloan Digital Sky Survey (SDSS) Data Release 7 spectroscopic catalog. We find 20,407 white dwarf spectra, representing 19,712 stars, and provide atmospheric model fits to 14,120 DA and 1011 DB white dwarf spectra from 12,843 and 923 stars, respectively. These numbers represent more than a factor of two increase in the total number of white dwarf stars from the previous SDSS white dwarf catalogs based on DR4 data. Our distribution of subtypes varies from previous catalogs due to our more conservative, manual classifications of each star in our catalog, supplementing our automatic fits. In particular, we find a large number of magnetic white dwarf stars whose small Zeeman splittings mimic increased Stark broadening that would otherwise result in an overestimated log g if fit as a non-magnetic white dwarf. We calculate mean DA and DB masses for our clean, non-magnetic sample and find the DB mean mass is statistically larger than that for the DAs.

New white dwarf stars in the Sloan Digital Sky Survey Data Release 10

We report the discovery of 9 089 new spectroscopically confirmed white dwarfs and subdwarfs in the Sloan Digital Sky Survey Data Release 10. We obtain Teff, log g and mass for hydrogen atmosphere white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), and estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon dominated spectra DQs. We found 1 central star of a planetary nebula, 2 new oxygen spectra on helium atmosphere white dwarfs, 71 DQs, 42 hot DO/PG1159s, 171 white dwarf+main sequence star binaries, 206 magnetic DAHs, 327 continuum dominated DCs, 397 metal polluted white dwarfs, 450 helium dominated white dwarfs, 647 subdwarfs and 6888 new hydrogen dominated white dwarf stars.

Magnetic white dwarf stars in the Sloan Digital Sky Survey

To obtain better statistics on the occurrence of magnetism among white dwarfs, we searched the spectra of the hydrogen atmosphere white dwarf stars (DAs) in the Data Release 7 of the Sloan Digital Sky Survey (SDSS) for Zeeman splittings and estimated the magnetic fields. We found 521 DAs with detectable Zeeman splittings, with fields in the range from around 1 MG to 733 MG, which amounts to 4% of all DAs observed. As the SDSS spectra has low signal-to-noise ratios, we carefully investigated by simulations with theoretical spectra how reliable our detection of magnetic field was.

Measuring the Evolution of the Most Stable Optical Clock G 117-B15A

We report our measurement of the rate of change of period with time () for the 215 s periodicity in the pulsating white dwarf G 117-B15A, the most stable optical clock known. After 31 years of observations, we have finally obtained a 4 σ measurement observed = (4.27 ± 0.80) × 10-15 s s-1. Taking into account the proper-motion effect of proper = (7.0 ± 2.0) × 10-16 s s-1, we obtain a rate of change of period with time of = (3.57 ± 0.82) × 10-15 s s-1. This value is consistent with the cooling rate in our white dwarf models only for cores of C or C/O. With the refinement of the models, the observed rate of period change can be used to accurately measure the ratio of C/O in the core of the white dwarf.

Direct measurement of a secular pulsation period change in the pulsating hot pre-white dwarf PG 1159-035

GW Vir (PG 1159(035) is the prototype of the class of multiperiodic, nonradially pulsating hot white dwarfs, and shows a strong pulsation mode at 516 s. All measurements to date of the secular variation of the 516 s pulsation quote as best value s s~1. The original measurement P0 \ ((2.49 ^ 0.06) ) 10~11 gave two best solutions, and a s2 analysis indicated that the quoted value was preferred at the level of 0.97 probability. On other hand, the best-developed models for planetary nebula nuclei (PNNs), using models from the asymptotic giant branch as starting points and simulating the observed mass loss, provide positive values for any model with as PG 1159(035. This con—ict between the log (L /L _ ) ( 3, measurement and the theoretical models has been a challenge to stellar evolution theory. Exploiting a much larger data set and computational techniques previously unavailable, we show that the earlier analysis of the data grossly underestimated the true uncertainties due to interferences between frequencies. Using new data along with the old, and more accurate statistical methods, we calculated the secular period change of the 516 s pulsation, and obtained a positive value: P0 \ ()13.07 ^ 0.03) ) 10~11 ss ~1. We show that three additional methods yield the same solution. This new value was the second best of the original possible solutions; it was eliminated on the basis of statistical arguments that we show to be invalid. It is an order of magnitude larger than the theoretical predictions. Additionally, from rotational splitting analysis, we were able to estimate, for the —rst time, a limit to the secular variation of the rotational period s s~1, leading to a contraction

Asteroseismological measurements on PG 1159-035, the prototype of the GW Virginis variable stars

Aims. An asteroseismological study of PG 1159−035, the prototype of the GW Vir variable stars, has been performed on the basis of detailed and full PG 1159 evolutionary models presented by Miller Bertolami & Althaus (2006, A&A, 454, 845). Methods. We carried out extensive computations of adiabatic g-mode pulsation periods on PG 1159 evolutionary models with stellar masses spanning the range 0.530 to 0.741 M� . These models were derived from the complete evolution of progenitor stars, including the thermally pulsing AGB phase and the born-again episode. We constrained the stellar mass of PG 1159−035 by comparing the observed period spacing with the asymptotic period spacing and with the average of the computed period spacings. We also employed the individual observed periods reported by Costa et al. (2007) to find a representative seismological model for PG 1159−035.

The temporal changes of the pulsational periods of the pre-white dwarf PG 1159-035

Context. PG 1159-035, a pre-white dwarf with Teff � 140 000 K, is the prototype of the PG 1159 spectroscopic class and the DOV pulsating class. Pulsating pre-white dwarf stars evolve rapidly: the effective surface temperature decreases rapidly, the envelope contracts and the inner structure experiences stratification due to gravitational settling. These changes in the star generate variations in its oscillation periods. The measurement of temporal change in the oscillation periods, u P, allows us to estimate directly rates of stellar evolutionary changes, such as the cooling rate and the envelope contraction rate, providing a way to test and refine evolutionary models for pre-white dwarf pulsating stars. Aims. Previously, only two pulsation modes of the highest amplitudes for PG 1159-035 have had their u P measured: the 516.0 s and the 539.3 s modes. We measured the u P of a larger number of pulsation modes, increasing the number of constraints for evolutionary studies of PG 1159-035. We attempted to use the secular variations in the periods of multiplets to calculate the variation in the rotational period, the envelope contraction rate, and the cooling rate of the star. Methods. The period variations were measured directly from the PG 1159-035 observational data and refined by the (O–C) method. Results. We measured 27 pulsation mode period changes. The periods varied at rates of between 1 and 100 ms/yr, and several can be directly measured with a relative standard uncertainty below 10%. For the 516.0 s mode (the highest in amplitude) in particular, not only the value of u P can be measured directly with a relative standard uncertainty of 2%, but the second order period change, ¨ P, can also be calculated reliably. By using the (O–C) method, we refined the u Ps and estimated the ¨ Ps for six other pulsation periods.

Pulsational frequencies of the eclipsing δ Scuti star HD 172189. Results of the STEPHI XIII campaign

Context. The eclipsing δ Scuti star HD 172189 is a probable member of the open cluster IC 4756 and a promising candidate target for the CoRoT mission. Aims. The detection of pulsation modes is the first step in the asteroseismological study of the star. Further, the calculation of the orbital parameters of the binary system allows us to make a dynamical determination of the mass of the star, which works as an important constraint to test and calibrate the asteroseismological models. Methods. We performed a detailed frequency analysis of 210 hours of photometric data of HD 172189 obtained from the STEPHI XIII campaign �� . Results. We have identified six pulsation frequencies with a confidence level of 99% and a seventh with a 65% confidence level of 65%, in the range between 100−300 µHz. In addiction, three eclipses were observed during the campaign, allowing us to improve the determination of the orbital period of the system.

A comparative analysis of the observed white dwarf cooling sequence from globular clusters

We report our study of features at the observed red end of the white dwarf cooling sequences for three Galactic globular clusters: NGC\,6397, 47\,Tucanae and M\,4. We use deep colour-magnitude diagrams constructed from archival Hubble Space Telescope (ACS) to systematically investigate the blue turn at faint magnitudes and the age determinations for each cluster. We find that the age difference between NGC\,6397 and 47\,Tuc is 1.98

Seismology of a massive pulsating hydrogen atmosphere white dwarf

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